Typical driver circuits comprise a source follower. A source follower circuit has the desired low output impedance. Source follower circuits, however, have an input-to-output voltage drop which makes them unsuitable for low voltage applications, as well as for driving large PMOS transistors. For example, if the output voltage needs to be within 500 mV of the positive voltage supply, a typical n-type source follower, or even an n-type emitter follower, may fail to provide the required drive voltage. Conversely, if a p-type source follower is used, the output voltage cannot be driven to within 500 mV of ground level, which is a typical requirement of low voltage applications (rail-to-rail circuits), not to mention linear regulators where the PMOS switch needs to be completely off in one extreme and sourcing large amounts of current in the other extreme, even with small input voltages. As an alternative, a differential amplifier in unity gain configuration can be used, but at the expense of complexity, die area, cost and speed/bandwidth.